In September 2010, the New Orleans District Corps of Engineers was trying to determine if a stage control structure built near Zachary, LA, in East Baton Rouge Parish was constructed according to design tolerances. The construction contractor's surveyor had collected 12,000 shots on the structure surface with a total station without being able to determine if it was within the strict tolerances required. Something more powerful was needed - and laser scanning turned out to be the answer.
The Amite River and its tributary, the Comite River, are the major causes of catastrophic flooding in the Baton Rouge metro area. In 2001, the U.S. Army Corps of Engineers, the state, the Amite River & Tributaries Study Authority and East Baton Rouge Parish signed an agreement to develop a regional solution called the Comite River Diversion Canal Project. Its purpose is to divert about 50% of the flood waters from the Upper Comite River to the Mississippi River.
Little room for deviation
Due for completion in 2016, the Comite River Diversion Canal Project involves construction of a 12-mile-long diversion channel from the Comite River to the Mississippi River; a diversion structure at the Comite River; guide levees; the Lilly Bayou stage control structure; and several drop structures where the diversion channel intersects with roads, railroad bridges and bayous. As of 2008, the total estimated cost of the project was $187 million.
The Lilly Bayou Control Structure will dissipate the energy of water flowing between the Comite and the Mississippi from a higher to lower elevation, according to Rick Tillman, structural engineer with the Corps. The energy will dissipate as water flows over a spillway featuring an elevated weir.
The $27.6 million second phase of the project consists of the excavation and construction of the concrete control structure, a stilling basin and an outflow channel. The main surface slab was designed to have a 1 to 5 (1v: 5h) slope, a several feet thick substructure of mass concrete and large baffle blocks filling the stilling basin at the bottom constructed of 3,000-psi concrete. A 1-foot-thick, 5,000-psi concrete overlay was also placed on top of the substructure to handle the compressive force and abrasion generated by rushing flood waters.
At no more than 1/8 inch of deviation for every 10 feet of surface, the elevation tolerance on the structure is uncommonly tight, notes Tillman. "That's out of the ordinary for the [Corps'] New Orleans District to design; however, it's not unusual for the purpose of the structure," he points out. "We have flow velocities that exceed 40 feet per second. At those velocities, you can get cavitation and if you have unevenness on the surface, it can tear a hole and tear the structure apart. That's why you have to have these tight tolerances."
After the structure was built, however, deviations from the elevation tolerance on the sloped face were plainly visible using a straight edge. Determining the extent to which the structure was out of tolerance promised to be quite a task in itself.
The difference: quantity of points
The Corps' lead engineering technician, Dwayne Blanchard, had previously experienced successful scanning projects completed for the division by Dale Stockstill & Associates (DS&A). He recommended that Tillman ask us about scanning the structure.
Eager to demonstrate the potential of scanning to the Corps, I agreed to perform the scan and data analysis without a task order. After obtaining the coordinates for the PK nails that were already in place from the construction company's project surveyor, our crew checked them with a GPT-3105W reflectorless total station from Topcon Positioning Systems, and then started the scan using a Topcon GLS-1000 laser scanner.
The elevation did not adhere to tolerance throughout the structure surface, but the X and Y axes were fairly close to tolerance. So the crew performed the scans from the existing control point, which was in place at the top and bottom of the large structure, knowing that the two scans could be corrected during the cloud registration process.